For such restrictions, here's a handy trick:
All of the newer Microsoft Office software should have this.
Get the two images that you want and insert them into say... Microsoft Word.
Align them up correctly (perhaps size/edit them as you want), then put them next to each other as you'd like to see in your signature.
Use the Group tool to put them together, essentially as one image.
Choose to save as a picture, upload it on a media hosting site, add it to your profile and bam.

There's of course the additional limit in the number of characters you can have in your signature, but that should be no issue, provided the URL isn't too long - most image hosting sites have fairly concise ones, at least.
I think the main reason for having such restrictions is that it may be an extra weight on the systems, and would take perhaps a bit longer to load each page if you have multiple pictures etc. on it.